FIXED: compiling with older gcc and used c standard

FIXED: compiling with older gcc and used c standard

With "gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4" we get the failure

    Scanning dependencies of target libMultiMarkdown
    [  3%] Building C object CMakeFiles/libMultiMarkdown.dir/Sources/libMultiMarkdown/aho-corasick.c.o
    /MultiMarkdown-6/Sources/libMultiMarkdown/aho-corasick.c: In function ‘ac_trie_node_prepare’:
    /MultiMarkdown-6/Sources/libMultiMarkdown/aho-corasick.c:283:2: error: ‘for’ loop initial declarations are only allowed in C99 mode
      for (int i = 0; i < 256; ++i)
      ^
    [...]

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>